Photo in sRGB with black and white parts converted to CMYK

Question

I know it's an often topic but haven't found any solution for my problem.

I`m working on a book with lamp photos - I was adjusting photos in Photoshop in sRGB mode. For instance, I have the lamp on a grey wall - in Photoshop I've made the wall colorless (with saturation set to 0%) and the lamp remains in color.

After that, I made a document in InDesign using such adjusted photos in sRGB color profile. Later I've exported a PDF file for print with color conversion set to preserve numbers and with the destination set to Coated Forga27 (a requirement set by the printer).

I've printed a proof with some photos and the grey wall appears a bit green-ish. So - finally - my question is: is there a successful way to convert to CMYK in which the grey remains grey? My suspicion is that the grey values from Photoshop are later translated into all CMYK values, not only a K value.

P.S. I found this post: How do I convert an RGB black and white photograph to just CMYK black

but that method used on my projects just deletes hue and saturation layers and changes the color of my whole project.

Answer 1

I think you have 2 problems here.

I've printed a proof

What do you mean by proof? If it is just a print in your home printer or office printer, there is no way of knowing if it is calibrated or not. Most likely not, so any grayscale image can potentially look any color.

A proof should be made in a calibrated environment, preferably by the same printer that is going to print your job. They can not guarantee color using a random print taken on another place.

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The second one is that if you can not have a neutral gray just saving an RGB image.

If it is going to be printed on a sheeted offset machine, you need to manually separate the plates and assign whatever values you want, in this case, probably k only.

This is accomplished by doing some masking. Keep the color part with the fogra profile conversion, and mask and change values of the background manually.

Be careful because that zone could potentially look artificial.

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P.S. White is a color, gray is a color, black is a color. They are not colorless, they are unsaturated.

Answer 2

What can help here is GCR (Grey Component Replacement).

Convert the images to a custom CMYK profile in Photoshop. There you can change settings for GCR color separation. This can result in a more neutral grey reproduction in print.

Answer 3

So first you didn't make the wall colorless. You made it appear without a color. It still has values in all three RGB channels (as only black is 0,0,0). You can't easily switch it to only K as it has values that can (and will) be placed in all CMYK channels.

You need to go and redo all photos in either all in CMYK OR separate the lamp and put a K background in InDesign (if they have really one color background it could be easily done with many "white remove" actions).